Cellular terminal location using GPS signals in the cellular band

ABSTRACT

Aspects of global positioning system (GPS) technology and cellular technology are combined in order to provide an effective and efficient position location system. In a first aspect of the invention, a cellular network is utilized to collect differential GPS error correction data, which is forwarded to a mobile terminal over the cellular network. The mobile terminal receives this data, along with GPS pseudoranges using a GPS receiver, and calculates its position using this information. According to a second aspect, when the requisite number of GPS satellites are not in view of the mobile terminal, then a GPS pseudosatellite signal, broadcast from a base station of the cellular network, is received by the mobile terminal and processed as a substitute for the missing GPS satellite signal. A third aspect involves calculating position using GPS when the requisite number of GPS satellites are in view of a GPS receiver, but when the requisite number of GPS satellites are not in view of the GPS receiver, then position is calculated using the cellular network infrastructure. When the requisite number of GPS satellites come back into view of the GPS receiver, then position is again calculated using GPS. A fourth aspect involves using cellular signals already being transmitted from base stations to terminals in a cellular network to calculate a round trip delay, from which a distance calculation between the base station and the terminal can be made. This distance calculation substitutes for a missing GPS satellite signal.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to global positioning satellite systemsand cellular networks, and in particular, combining aspects of eachtechnology in order to provide an efficient, reliable, and highlyaccurate position location system.

BACKGROUND OF THE INVENTION

The NAVSTAR (Navigation System with Time and Range) Global PositioningSystem (GPS) is a space-based radio-positioning and time-transfersystem. While the system was originally developed primarily for militarypurposes, it now also contains a “coarse acquisition” (C/A) channel thatis available for general civilian use. GPS provides accurate position,velocity, and time (PVT) information for a given object anywhere on theface of the earth, such as a moving mobile terminal in a vehicle. TheNAVSTAR GPS includes three major system segments: (i) a space segment,(ii) a control segment, and (iii) a user segment. Briefly, the spacesegment has twenty four NAVSTAR satellites, each of which broadcastsradio frequency (RF) ranging codes and navigation data messages. Eachnavigation data message includes such data as satellite clock-bias data,ephemeris data (precise orbital data of the satellite), certaincorrection data, and satellite almanac data (coarse orbital data on the24 satellites). The twenty four satellites are arranged in six orbitalplanes with four satellites in each plane, and the orbital planes areinclined at an angle of 55 degrees relative to the earth's equator. Thecontrol segment primarily consists of a master control station currentlyat Falcon Air Force Base in Colorado, along with monitor stations andground antennas at various locations around the world. The mastercontrol station monitors and manages satellite constellation. Themonitor stations passively track GPS satellites in view and collectranging data for the satellites. This ranging data is transmitted to themaster control system where satellite ephemeris and clock parameters areestimated and predicted. Furthermore, the master control system uses theground antennas to periodically upload the ephemeris and clock data toeach satellite for retransmission in the navigation data message.Finally, the user segment comprises GPS receivers, specially designed toreceive, decode, and process the GPS satellite signals.

Generally, the satellites transmit ranging signals on two D-bandfrequencies: Link 1 (L1) at 1575.42 MHz and Link 2 (L2) at 1227.6 MHz.The satellite signals are transmitted using spread-spectrum techniques,employing ranging codes as spreading functions, a 1.023 MHz coarseacquisition code (C/A-code) on L1 and a 10.23 MHz precision code(P-code) on both L1 and L2. The C/A-code consists of a 1023 bitpseudorandom (PRN) code, and a different PRN code is assigned to eachGPS satellite, as selected from a set of codes called Gold codes. TheGold codes are designed to minimize the probability that a receiver willmistake one code for another (i.e., minimize cross-correlation). TheC/A-code is available for general civilian use, while the P-code is not.In addition, a 50 Hz navigation data message is superimposed on theC/A-code, and contains the data noted above.

In particular, the navigation message has 25 frames of data, each framehaving 1,500 bits. Each frame is divided into five subframes of 300 bitseach. At the 50 Hz transmission rate, it takes six seconds to receive asubframe, thirty seconds to receive one data frame, and 12.5 minutes toreceive all twenty five frames. Subframes 1, 2, and 3 have the same dataformat for all twenty five frames. This allows the receiver to obtaincritical satellite-specific data within thirty seconds. Subframe 1contains the clock correction for the transmitting satellite, as well asparameters describing the accuracy and health of the broadcast signal.Subframes 2 and 3 contain ephemeris parameters. Finally, subframes 4 and5 contain data common to all satellites and less critical for a receiverto acquire quickly, namely almanac data and low-precision clockcorrections, along with other data.

The ranging codes broadcast by the satellites enable the GPS receiver tomeasure the transit time of the signals and thereby determine the rangebetween the satellite and the receiver. It should be noted, however,that range measurements inherently contain an error called an offsetbias common to all the measurements created by the unsynchronizedoperation of the satellite and the user's clocks. See U.S. Pat. No.5,467,282 to Dennis. This user clock error will yield an erroneous rangemeasurement, making it appear that the user is either closer to orfarther from each of the satellites than is actually the case. Thesemeasurements are therefore more accurately termed pseudoranges. Thenavigation data messages enable the receiver to calculate the positionof each satellite at the time the signals were transmitted.

In general, four GPS satellites must be in clear view of the GPSreceiver in order for the receiver to accurately determine its location.The measurements from three GPS satellites allow the GPS receiver tocalculate the three unknown parameters representing itsthree-dimensional position, while the fourth GPS satellite allows theGPS receiver to calculate the user clock error, and therefore determinea more precise time measurement. The GPS receiver compiles thisinformation and determines its position using a series of simultaneousequations.

In addition, when the GPS receiver is first turned on, it must calculateits initial position. This initial determination is known as a “firstfix” on location. Typically, the receiver must first determine whichsatellites are in clear view for tracking. If the receiver is able toimmediately determine satellite visibility, the receiver will target asatellite and begin its acquisition process. If there is no almanac orposition information already stored in the receiver, then the GPSreceiver enters a “search the sky” operation that searches forsatellites. Once the satellites are tracked, the receiver beginsreceiving the necessary data, as described above.

The “time-to-first-fix” (TTFF) represents the time required for areceiver to acquire the satellite signals and navigation data, and tocalculate its initial position. If the receiver has no estimate ofcurrent time and position and a recent copy of almanac data, then thisprocess generally takes about 12.5 minutes, which is the time necessaryto receive a complete navigation data message assuming a 50 Hztransmission rate and receipt of twenty five frames of data, asdescribed above.

A common problem with the conventional GPS is not having four GPSsatellites in clear view of the GPS receiver. This commonly arises, forexample, in a city setting such as in an urban canyon—i.e., in theshadow of a group of tall buildings—which can block the GPS satellitesignals, or indoors in the buildings themselves. In such situations, theGPS receiver is unable to accurately determine its location using GPS.

Therefore, the need arises to find a replacement for the one or moremissing GPS satellite signals. One method for accommodating this problemis to provide pseudosatellite signals that are transmitted in the GPSfrequency band. They provide much the same information that the typicalGPS satellite does, and are utilized by the GPS receiver in much thesame fashion as the typical GPS satellite signal. These signals mayoriginate from dedicated stations that are located on the ground atstrategic locations, such as at airports. However, pseudosatellitesignals are stronger than the GPS satellite signals and therefore, blockthe GPS signals. Thus, they generally transmit for only ten percent ofthe time. That is, they transmit periodically, known as burst mode, suchas on for ten percent of the time and off for ninety percent of thetime.

In addition to drowning out actual GPS satellite signals, theconventional pseudosatellite signal approach has other disadvantages.For one, there is the need to have specialized dedicated stations atstrategic locations to transmit this information. This increases thecost of the GPS, and requires the need for obtaining permission from thelandowner to set up and operate such dedicated stations. In addition,the user must be located within some specified distance of the stationin order to receive the pseudosatellite signal, which is not always thecase. Therefore, there is a need for a more efficient, less costly, andreliable alternative for addressing the situation of an inadequatenumber of GPS satellites being in clear view of the GPS receiver.

In addition, even when four satellites are in view, and the GPS receiveris readily receiving all of the necessary pseudorange data forcalculating its position, there are further common errors present thatresult in erroneous position determinations. These errors includephysical errors such as signal path delays through the atmosphere, i.e.,propagation signal delay, and satellite clock and ephemeris errors. Inaddition, for civilian users, the Government introduces errors fornational security reasons, generally known as selective availabilityerrors (SA). SA primarily includes ephemeris data error and clock error,and results in an erroneous position determination of approximately 25to 100 meters.

In order to help reduce the effects of these errors, a differential GPS(DGPS) may be employed. DGPS can achieve accuracies in the order of tenmeters. The typical DGPS architecture includes one or more referencestations at precisely known, fixed reference sites, and DGPS receivers.The reference station includes a reference receiver antenna, adifferential correction processing system, and data link equipment. Asan example, the United States Coast Guard has set up reference stationsthat broadcast the differential correction data, which is typically usedby ships.

There are two primary variations of the differential measuringtechniques. One technique is based on ranging-code measurements and theother is based on carrier-phase measurements. In general, theranging-code differential technique uses the pseudorange measurements ofthe reference station to calculate pseudorange or position correctionsfor the user receivers. The reference station calculates the pseudorangecorrections for each visible satellite by subtracting the “true” range,determined by surveyed position and the known orbit parameters, from themeasured pseudorange. The reference station typically broadcasts thepseudorange corrections in real-time on a low frequency beacon channel,which is received in real-time by the DGPS receiver. Of course, both theDGPS receiver and the reference receivers could alternatively collectand store the necessary data for later processing. The DGPS receiverselects the appropriate correction for each satellite that it istracking, and subtracts the correction from the pseudorange that it hasmeasured. For example, with the reference station set up by the CoastGuard, the station will broadcast the pseudorange corrections as radiosignals. Ships having DGPS receivers receive this radio signal andprocess it to correct the pseudorange data obtained from the GPSsatellites.

The other differential technique is the carrier-phase differentialtechnique, which is typically used in applications requiring highaccuracy such as in surveying or for an aircraft landing system. Thismethod measures the difference in phase of the carrier at the referenceand mobile unit. The ambiguity in the integer number of cycles isdetermined by either bringing the antennae of the reference unit andmobile unit close together (less than one wavelength), or by redundantmeasurements and complex search algorithms to determine the correctsolutions.

Furthermore, DGPS may be designed to serve a limited area from a singlereference station, which is generally called a local area DGPS (LADGPS).In the alternative, the system may use a network of reference stationsand known algorithms to extend the validity of the DGPS technique over awide area—known as Wide Area GPS, or WADGPS.

The typical DGPS presents certain drawbacks. One drawback is that theDGPS must use its own frequency band, so as not to interfere with thatof the stand alone GPS. In addition, the DGPS receiver presents anadditional receiver that must operate independent of the GPS receiversin receiving the differential correction data. These problems work indirect tension with the desire to make such systems as small and compactas possible, with as little additional circuit structure as possible,and still be as efficient as possible in terms of utilizing limitedfrequency.

Another area of interest for the present invention is cellulartechnology. FIGS. 1 and 2 show a typical cellular network, and its maincomponents. See U.S. Pat. No. 5,546,445 to Dennison et al. The typicalcellular network 100 covers a contiguous area that is generally brokendown into a series of cells 110. Each cell has a base station 210 thatmaintains communication with the mobile terminal 220 (e.g., a cellularphone). The base station 210 includes a transmitter and receiver (ortransceiver), and an antenna that transmits a wireless signal over agiven area. The transmit power of the base station is directly relatedto the size of the cell, where the greater the transmit power of thebase station, the larger the size of the cell.

The overall management of the cellular system is handled by a mobiletelecommunications switching office (MTSO) 120. The MTSO providesnumerous functions for the cellular system, such as assigning calls to acell based on availability and signal strength, call statistics, andbilling for the cellular network. The MTSO also functions as theinterface between the cells and the Public Telephone Switching Network(PTSN) 140 for connection to the local telephone company 230 and longdistance toll centers.

In configuring the cellular network, the desired size of the celldepends on the geographic nature of the coverage area and the amount oftraffic expected in that area. Each cell uses a group of assignedfrequencies or channels. In addition, where traffic becomes too heavy ina given area, the cell may be split into smaller cells by a processknown in the art as “cell splitting.” This concept is generallyillustrated in FIG. 1.

In many instances, a cellular user also wishes to determine theirlocation. The cellular user may carry around a GPS receiver fordetermining location. An alternative is to have the GPS receiverincorporated into the cellular mobile terminal. See, for example, U.S.Pat. No. 5,043,736 to Darnell et al and U.S. Pat. No. 5,625,668 toLoomis et al. Methods also exist for determining location in a cellularsystem independent of GPS in order to determine location, such as usingthe cellular network infrastructure. Two examples for calculatingposition (though not the only methods) are (i) using Time Of Arrival(TOA) measurements when the time of transmission of the signal from thebase stations is known, or (ii) using Time Difference of Arrival (TDOA)measurements when the actual time of transmission is not known, butperiodic signals are available, as explained below.

Referring generally to FIG. 3, a typical urban street pattern 300 isshown to illustrate the first method of using TOA measurements. When thetime of transmission of the signal from a base station 310 is known, amobile terminal 320 simply determines when that transmitted signal isreceived. The difference in time from transmission to receipt, alsoknown as the propagation delay, multiplied by the speed of light,provides a radial distance measurement R between that base station andthe mobile terminal. Calculating the distance between the mobileterminal and three different base stations provides an accurate locationfix for the mobile terminal, as the intersection of three spheres.

In the second method of utilizing TDOA measurements, while this approachcan also be used when the actual time of transmission of the signal fromthe base stations is available, it may also be used when such time oftransmission is not available, but periodic signals are. This may occurwith some cellular systems. Some CDMA (Code Division Multiple Access)systems, such as those conforming to the IS-95 standard, do providetransmissions at well defined times.

The periodic signal entails each of the base stations transmittingperiodic signals that are synchronized with one another. In that regard,all of the base stations may transmit their periodic signals at the sameexact time, or with some specified timing offset between base stations.In this method, the mobile terminal measures the difference in timebetween the arrival of a signal from one base station with respect toanother. This time difference of arrival (TDOA), together with the knownlocations of the two base stations and the speed of radio signaltransmission, defines a hyperbolic surface with the base stations at thefoci. The mobile terminal's location is somewhere on this surface. Thus,a single TDOA measurement does not uniquely define the location of themobile terminal. However, a similar measurement for signals from otherpairs of base stations defines additional surfaces. By measuring theTDOA of the signals from three base stations, three surfaces can bedetermined, the common intersection of which establishes the location ofthe mobile terminal.

Further information and systems regarding conventional TDOA locationsystems and methods may be found in Krizman et al., “Wireless PositionLocation Fundamentals, Implementation Strategies, and Sources of Error,”presented at the IEEE Conference on Vehicular Technology, Phoenix,Ariz., May 5-7, 1997 and in the issue of the IEEE CommunicationsMagazine, April 1998, Vol. 36, No. 4, pages 30-59. The entirety of thisreference is hereby incorporated into the present disclosure for itsteachings regarding conventional TDOA location methods and systems.

However, problems exist with using these two methods for determininglocation. One significant problem results from multi-path errors. Sucherrors result from changes in the transmission path of the signal thatthe mobile terminal receives from the base station. For example, whenthe user of the mobile terminal goes around a corner, the mobileterminal may receive a new signal from the base station that hasfollowed a completely different transmission path compared to the oldsignal that the mobile terminal was previously receiving before the userturned the corner. Therefore, the distance traveled by the signal willlikely differ. This causes a change in time measurement by the mobileterminal that does not accurately represent the actual distance changeof the mobile terminal from the base station, thereby rendering aninaccurate location determination by the mobile terminal.

Another problem encountered is that the typical clock in a cellularmobile terminal does not measure time precisely, and may have a tendencyto drift, generally known as clock drift. Therefore, the timemeasurements made by the terminal are not extremely accurate, whichresults in an erroneous time—and therefore location—determination. Theerror due to the drift grows larger the longer the mobile terminal clockis used.

In sum, as shown above, a need exists for a more efficient and lesscostly structure compared to the conventional DGPS receiver. Inaddition, a need exists for more efficient, reliable, and effectivesolutions to address the problem of receiving an inadequate number ofsatellite signals from the GPS satellites.

SUMMARY OF THE PRESENT INVENTION

It is an object of the present invention to provide a system thatcombines GPS and cellular technology in order to overcome deficienciesassociated with the use of either technology alone, in order to providea more efficient, reliable, and effective position determination for agiven object such as a mobile terminal.

It is a further object of the present invention to provide a positionlocation system that utilizes the cellular network to forward DGPS errorcorrection information to a mobile terminal.

It is yet another object of the present invention to provide a positionlocation system that efficiently utilizes the cellular frequency bandavailable in a cellular network for forwarding DGPS error correctioninformation to a mobile terminal.

It is a further object of the present invention to provide a positionlocation system that provides a cellular network with the capability ofreceiving and forwarding DGPS error correction information that isutilized by the mobile terminal for accurately determining its position.

It is another object of the present invention to provide a positionlocation system that provides a cellular network with the capability ofreceiving and forwarding DGPS error correction information to a DGPSprocessor, and also forwarding GPS pseudoranges from the mobile terminalto the DGPS processor, wherein the GPS pseudoranges are corrected.

It is yet another object of the present invention to provide a positionlocation system that compensates for the inability of a mobile terminalcontaining a GPS receiver to view the requisite number of GPS satellitesto obtain an accurate fix on its location.

It is yet a further object of the present invention to provide aposition location system that utilizes the base station of a cellularnetwork to transmit GPS pseudosatellite signals such that if therequisite number of GPS satellites are not in clear view of the mobileterminal containing the GPS receiver, the mobile terminal can accuratelydetermine its position using both cellular-based pseudosatellite signalsand available GPS signals.

It is another object of the present invention to provide a positionlocation system that includes a base station in a cellular network thatis capable of generating and transmitting GPS pseudosatellite signalsindependent of receiving GPS signals.

It is a further object of the present invention to provide a positionlocation system that includes a mobile terminal capable of receiving aGPS pseudosatellite signal from a base station of a cellular network,and processing that signal as a substitute for a missing GPS satellitesignal, and in combination with available GPS satellite signals, todetermine position.

It is another object of the present invention to provide a positionlocation system that efficiently makes use of both a position locationscheme using the cellular network infrastructure and a GPS locationsystem when the requisite number of GPS satellites are not in clear viewof the mobile terminal containing the GPS receiver.

It is a further objective of the present invention to provide a positionlocation system that makes use of information from both the GPS and thecellular network infrastructure to provide improved accuracy andreliability than could be achieved by either system working alone.

It is yet a further object of the present invention to provide aposition location system that makes use of a position location schemeusing the cellular network infrastructure and a GPS location system asappropriate to minimize power consumption in the terminal.

It is a further object of the present invention to provide a positionlocation system that efficiently switches to a position location schemebased on a combination of using the cellular network infrastructure andthe available GPS satellite signals when the requisite number of GPSsatellites are not in clear view of the mobile terminal containing theGPS receiver.

It is yet another object of the present invention to provide a positionlocation system that converts from a position location scheme using thecellular network infrastructure to GPS when the requisite number of GPSsatellites are in clear view of the mobile terminal containing the GPSreceiver.

It is yet a further object of the present invention to provide aposition location system that utilizes GPS technology to reduce theproblems associated with the position location scheme that uses thecellular network infrastructure.

It is another object of the present invention to provide a positionlocation system that reduces the effects of radio multi-path propagationand clock drift associated with position location schemes using cellularnetwork infrastructure by utilizing GPS technology.

It is yet a further object of the present invention to provide aposition location system that efficiently utilizes cellular signals of aCDMA or TDMA system to augment the position determination.

In order to achieve these and other objects, the present inventionprovides a position location system that incorporates particular aspectsof the cellular network with GPS. For one, in the position locationsystem of the present invention, the cellular network is utilized tocollect DGPS error correction information, and forward it to the mobileterminal over the established cellular network in the cellular band. Themobile terminal includes a DGPS processor that processes theinformation, along with pseudoranges received from a GPS receiver, inorder to calculate a more precise position than that obtained from GPSstanding alone. In the alternative, the DGPS processor is connected to acommunications network which is also connected to the base station, andreceives DGPS error correction data, along with the pseudoranges fromthe GPS receiver, over the cellular network (from the mobile). The DGPSprocessor uses this information to correct the pseudoranges to obtainmore accurate ranges.

Second, in the position location system of the present invention, whenthe requisite number of GPS satellites are not in view of the GPSreceiver, the system utilizes a GPS pseudosatellite signal that isgenerated by one or more base stations of the cellular networkindependent of the GPS, that is, independent of having to receive GPSsignals at the base station. The base stations are modified to generateand broadcast such pseudosatellite signals, and the pseudosatellitesignal is received and processed by the mobile terminal as a substitutefor an actual GPS satellite signal. Processing this information alongwith the satellite signal information from the GPS satellites that arein clear view, the mobile terminal is able to determine its positionmore accurately.

Third, in the position location system of the present invention, whenthe requisite number of GPS satellites are not in clear view of the GPSreceiver, the system switches from relying on the GPS portion of thesystem to utilizing cellular network infrastructure to determinelocation. This can be done, for example, by using either the TOA or TDOAmethods for determining location in a cellular network portion of thesystem. Furthermore, when the mobile terminal is moved to a locationwhere the requisite number of satellites are again in clear view of theGPS receiver, the system efficiently switches back to using the GPSportion of the system to determine location. An alternative is to use acombination of GPS satellite signals and cellular signals from basestations to calculate position.

Fourth, the cellular signals already transmitted, for example, in a CDMAor TDMA (Time Division Multiple Access) system may be used as areplacement of a missing GPS signal or to augment and improve GPSmeasurements. Using either the CDMA or TDMA system, a round trip delayis calculated with respect to a base station, from which the radius ofthe terminal from the base station is calculated. Further calibrationcan be achieved by calculating a timing offset correction, to achieve amore accurate radius measurement.

Other objects, features, and advantages of the present invention willbecome apparent from reading the following Detailed Description inconjunction with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 illustrates a conventional cellular network area divided into aplurality of cells.

FIG. 2 illustrates the major components of a conventional cellularnetwork scheme.

FIG. 3 illustrates the concept of determining position of a mobileterminal based on Time of Arrival measurements in a cellular networksystem.

FIG. 4 is a block diagram of a mobile telecommunications switchingoffice (MTSO) and base station for implementing a first aspect of theposition location system of the present invention.

FIG. 4A is a block diagram of a communications network and base stationfor implementing a first aspect of the position location system of thepresent invention.

FIG. 5 is a block diagram of a mobile terminal for carrying out thefirst aspect of the position location system of the present invention.

FIG. 6 is a block diagram of an alternative embodiment of the MTSO andbase station for implementing the first aspect of the position locationsystem of the present invention.

FIG. 6A is a block diagram of an alternative embodiment of acommunications network and base station for implementing a first aspectof the position location system of the present invention.

FIG. 7 is a block diagram of an alternative embodiment of the mobileterminal for carrying out the first aspect of the position locationsystem of the present invention.

FIG. 8 is a block diagram of a base station for implementing a secondaspect of the position location system of the present invention.

FIG. 9 is a block diagram of a mobile terminal for carrying out thesecond aspect of the position location system of the present invention.

FIG. 10 is a block diagram of a mobile terminal and base stationsaccording to a third aspect of the position location system of thepresent invention.

FIG. 11 is a flowchart depicting operation of the position locationsystem according to the third aspect of the present invention.

FIG. 12 is a flowchart depicting operation of the position locationsystem according to an alternative of the third aspect of the presentinvention.

FIG. 13 is a flowchart depicting operation of the position locationsystem according to a further alternative of the third aspect of thepresent invention.

FIG. 14 is a block diagram of a base station for implementing a fourthaspect of the position location system of the present invention.

FIG. 15 is a flowchart depicting carrying out a correction calculationaccording to the fourth aspect of the position location system of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The various aspects of a position location system according to thepresent invention are described below.

First Aspect

Referring to FIGS. 4 and 5, a first aspect of a position location systemof the present invention will be described. Broadly, FIG. 4 shows asource for DGPS error correction data 400, a cellular mobiletelecommunications switching unit 410, and a base station 440. FIG. 5shows a mobile terminal 500, which is typically at a remote locationrelative to the cellular mobile telecommunications switching unit and inthe transmitting vicinity of the base station 440. The transmittingvicinity is the area over which the base station broadcasts its signals.In general, this aspect of the invention involves the use of thecellular network to transmit DGPS error correction data to the mobileterminal, where it is used to perform corrections on pseudorange dataalso received at the mobile terminal.

First, in FIG. 4, the source 400 is responsible for providing DGPS errorcorrection data (i.e., differential error correction data). Numerousalternatives for source 400 exist to provide such information, includingusing Government sources, commercial operators, or the cellularoperator. For example, the source 400 may be a Government source, suchas the Coast Guard, which broadcasts DGPS error correction data as radiosignals from reference stations that it has established. Alternatively,a commercial supplier may be used to supply the DGPS error correctiondata. Two examples of such commercial suppliers are DifferentialCorrection, Inc. (DCI) of California, and Omnistar, Inc. of Texas. Inparticular, DCI currently uses FM radio stations to broadcast thecorrection information while Omnistar uses a geostationary satellite tobroadcast the correction information.

A third alternative is that the cellular provider set up its ownreference stations that calculate the pseudorange corrections for eachvisible satellite and broadcast them over the cellular network. In thatcase, the reference stations may be part of the base stations in thecellular network system.

For any particular application, depending on which alternative isselected for use as the source 400, the source will contain anappropriate receiver, such as a satellite receiver, an FM receiver, abeacon receiver, etc. For example, if Omnistar were used as the sourcethen a satellite receiver would be necessary. It should be noted thatthe signal received will be in the satellite frequency band. The typicaloverall satellite frequency band includes approximately 1200-1600 MHzand 3500-4300 MHz. The Omnistar system, for example, uses the 1551.489,1554.497 and 1556.825 MHz frequencies for its coverage of the UnitedStates.

Furthermore, the present invention contemplates a system that has two ormore of the above-noted sources available, and obtaining the informationfrom one or more of those sources as desired. For example, the systemmay have the ability to receive DGPS error correction information fromboth a commercial supplier and reference stations set up by the cellularprovider. In that circumstance, the source 400 would include circuitrythat would decide which source to utilize, based on, for example,availability of each source.

In addition to receiving the differential error correction data, thesource 400 will generally also convert the data into a standard DGPSsignal, such as, for example, as defined by RTCM SC-104 (Radio TechnicalCommission for Maritime Services, Special Committee-104), which hasdeveloped international standards for digital messaging. The DGPS signalis forwarded over a data link 405 to the cellular mobiletelecommunications switching unit 410, which is here a modified versionof a mobile telecommunications switching office (MTSO) known in the art.Data link 405 may be any type known in the art and compatible for usewith the MTSO 410.

Furthermore, relevant to this aspect of the invention, the MTSO 410includes a processing unit 415, a central unit 420 that is capable ofreceiving data and messages from other sources, a multiplexer 425, and aswitching unit 430. Processing unit 415 is responsible for convertingthe DGPS signal into a proper format for further transmission over thecellular network. For example, processing unit 415 may convert thereceived signal into a short message by using a short message service(SMS) as defined in the Global System for Mobile Communication (GSM)standard. GSM represents a mobile cellular system as defined by a set ofoperating standards, as introduced by the European body ETSI. For thepurposes of understanding the present invention, a short message isessentially a data packet containing the DGPS signal.

Central unit 420 contains other data and message sources that aresupplying information that must be forwarded by the MTSO 410. Suchinformation would include other short messages intended for transmissionto the same base station, voice data, data traffic for users havingmodems at the mobile terminal, and the like. This other information inthe central unit 420 is combined with the short message containing thedifferential error correction data using multiplexer 425 to create acombined signal.

Thereafter, the combined signal is transmitted to a switching unit 430of the MTSO 410 which is responsible for switching the data to anappropriate data link 435 for forwarding to one of numerous cellularnetwork base stations in the cellular system. Here, the intended basestation for receiving the combined signal is represented by referencenumeral 440. Base station 440 includes a base station modulator 445 thatmodulates the signal, and then transmits it through a radio interface450 and a base station antenna 455. It should be noted that thetransmission of the signal by the base station will be in the cellularfrequency band. The typical overall cellular frequency band includesapproximately 800-900 MHz and 1850-1990 MHz.

An analogous FIG. 4A shows the preferred arrangement. In thisarrangement the source of DGPS data 400 is connected to a communicationsnetwork 460 by a data link 406. In this view the source of DGPS data maybe, for example, a workstation or server attached to the Internet andproviding DGPS data for many base stations in one or more mobilenetworks. Although this server provides a logically separate function,it may be combined with, or physically located in conjunction with, apart of the communications network (for example such as an MTSO). Itshould be noted that the communications network may simply be an MTSO(as shown in FIG. 4), include an MTSO or a plurality of MTSOs with othercomponents, or may itself not include any MTSOs. The server may also beoperated by a third party, separate from the mobile network operator,and located remotely from the communications network components. Thebase station 440 is also connected to the communications network throughdata link 436. The communications network interconnects the source ofDGPS data and the base station and provides the similar functions to theMTSO shown in FIG. 4 of receiving messages from the DGPS source andcombining these together with other data and messages destined for thebase station for transmission to the mobile terminals served by the basestation. Such a communications network, for example, is provided by theInternet and the associated Internet protocols (IP) for addressing,formatting, sending and receiving messages to devices attached to thenetwork.

The signal containing the short message with the DGPS error correctiondata is transmitted as a radio frequency signal in the cellular band tothe mobile terminal 500, which is shown in FIG. 5. Mobile terminal 500generally includes a cellular antenna 505, a radio interface 510, adigital signal processor (DSP) 512, a central processor 515, a DGPSprocessor 520, a control unit 525, a speaker 530, and a GPS receiver 550having a GPS antenna 555. Mobile terminal 500 receives the radiofrequency signal via the cellular antenna 505, and forwards it to theradio interface 510. Radio interface 510 converts that signal into anintermediate frequency (IF) signal that is compatible with the DSP 512,and forwards the signal to the DSP 512. The DSP 512 demodulates thesignal into a data stream and forwards that data stream to the centralprocessor 515. Central processor 515 is responsible for determining thecontents of the data stream and forwarding the appropriate portions ofthe data stream to their intended destinations. For example, a typicaldata stream may contain short messages, one of which is the DGPS errorcorrection data, control data, voice data, and other information.

Regarding the short message carrying DGPS error correction data, thecentral processor 515 utilizes the protocol of the short message, whichuse of protocol is known in the art, to extract the error correctiondata out of the signal and forward it to the DGPS processor 520. Inaddition, for illustrative purposes, the central processor 515 forwardscontrol data to the control unit 525, while voice data may be processedand forwarded to the speaker 530. FIG. 5 does not depict all of thevarious destinations for all types of data encountered by the centralprocessor 515.

In addition, the GPS receiver 550 receives satellite signals by the GPSantenna 555 from the GPS satellites that are in view, and calculates thepseudoranges between the mobile terminal and each of the GPS satellites.GPS receiver 550 forwards these pseudoranges to the DGPS processor 520.

DGPS processor 520 utilizes the DGPS error correction data in aconventional manner to correct the calculated pseudoranges. Thecorrected ranges are forwarded back to the GPS receiver 550, which thenutilizes the information to calculate a more accurate position of themobile terminal 500 than could be achieved by using GPS standing alone.

It should be noted that the cellular antenna 505 and the GPS antenna 555may be formed as a single antenna that is capable of receiving bothtypes of signals, and may use, for example, an amplifier that respondsto both cellular and satellite bands.

An alternative structure for carrying out the correction of thepseudoranges is shown by FIGS. 6, 6A and 7. Much of the structureremains similar as in the embodiment shown in FIGS. 4, 4A and 5, forwhich analogous reference numerals have been used. However, in thisembodiment, the mobile terminal 700 has no DGPS processor. Rather, aDGPS processor 675 is connected to a source of DGPS data 600 by a datalink 605, and is connected by another data link 612 to a processing unit615 (which may be a multitude of processors) in a MTSO 610.

The remaining elements are a central unit 620, a multiplexer 625, and aswitching unit 630 in the MTSO 610, a data link 635, and a base station640 that includes a base station modulator 645, a radio interface 650and a base station antenna 655.

An analogous FIG. 6A shows the preferred arrangement. In thisarrangement the source of DGPS data 600 is connected to a communicationsnetwork 660 by a data link 606. In this view the source of DGPS data maybe, for example, a workstation or server attached to the Internet andproviding DGPS data to one or more base stations in one or more mobilenetworks. Similarly the DGPS processor 675 is also attached to thecommunications network through data link 614. In this view the DGPSprocessor may also be, for example, a server attached to the Internetand providing DGPS processing services for one or more base stations andtheir associated mobile terminals in one or more mobile networks. Thebase station 640 is also connected to the communications network throughdata link 636. The communications network interconnects the source ofDGPS data, the DGPS processor and the base station and provides thesimilar functions to the MTSO shown in FIG. 6 of receiving messages fromthe DGPS source and combining these together with other data andmessages destined for the base station for transmission to the mobileterminals served by the base station. Similarly the communicationsnetwork provides the function of transporting messages reporting GPSpseudoranges received by the base station from the mobile terminals tothe DGPS processor. Such a communications network, for example, isprovided by the Internet and the associated Internet protocols (IP) foraddressing, formatting, sending and receiving messages to devicesattached to the network.

Furthermore, the mobile terminal 700 contains a cellular antenna 705, aradio interface 710, a digital signal processor (DSP) 712, a centralprocessor 715, a control unit 725, a speaker 730, and a GPS receiver 750having a GPS antenna 755.

In this embodiment, the GPS receiver 750 receives satellite signals bythe GPS antenna 755 from the GPS satellites that are in its view andcalculates the pseudoranges between the mobile terminal 700 and each ofthe GPS satellites. The results of this determination, i.e., thecalculated pseudoranges, are then forwarded through the system to theDGPS processor 675 as described below. It should be noted that this maybe done automatically in a continuous fashion, i.e., each time the GPSreceiver 750 calculates pseudoranges it also forwards the pseudorangesto the DGPS processor 675. Alternatively, it may be done upon a specificrequest to the GPS receiver 750.

Forwarding of the calculated pseudoranges to the DGPS processor 675 maybe carried out by forwarding the calculated pseudoranges to the centralprocessor 715, which multiplexes the calculated pseudoranges into a datastream that is forwarded to the DSP 712. The DSP 712 modulates the datastream into an IF signal and forwards the signal to the radio interface710. The radio interface 710 converts the IF signal into a radiofrequency signal having a frequency that is in the cellular band. Thecellular signal is transmitted to be received at the base station 640.Base station 640 demodulates the signal, and forwards the demodulatedsignal message through the data links to the DGPS processor 675. If thesignal message passes through data link 635 to the MTSO 610, the MTSOextracts the calculated pseudorange messages, for example, by using theprocessing unit 615, and forwards them over the data link 612 to theDGPS processor 675. If the signal message passes through data link 636to a communications network, it will be received by the DGPS processorthrough data link 614 where the relevant pseudorange messages may beextracted.

In addition, the source 600 provides a signal containing the DGPS errorcorrection data to the DGPS processor 675. Therefore, the DGPS processor675 will have the pseudoranges as calculated by the GPS receiver 750,along with the necessary error correction data from the source 600. DGPSprocessor 675 performs the required corrections on the calculatedpseudoranges and transmits the corrected ranges back through the systemto the GPS receiver 750. GPS receiver 750 then calculates a moreaccurate position of the mobile terminal 700 using these correctedpseudoranges.

In the alternative, DGPS processor 675 could calculate the correctedposition of the mobile and forward the position to the mobile fordisplay, or to the network service or other service requesting theposition. The requesting service may be a process of the cellularnetwork itself, or it may be a server operated by a third partyconnected to the communications network.

It should be noted that in either of the embodiments shown in FIGS. 4(4A), 5 and FIGS. 6 (6A),7, one or more of the elements may beintegrally formed with other elements, i.e., be formed as part of asingle unit. For example, the DGPS processor may be integral with thecentral processor or the GPS receiver of the mobile terminal. Inaddition, while not described in detail, the DGPS processor mayalternatively be located at the base station or other locations andconnected to the base station through the communications network. Inaddition, the first aspect of the position location system of thepresent invention may be used with either a local area DGPS (LADGPS)system or a wide area DGPS (WADGPS) system.

This aspect of the invention provides numerous advantages over theconventional systems. For one, there is no need for a separate beaconreceiver at each mobile terminal for receiving the DGPS error correctioninformation from the source. That is, in the conventional DGPS approach,each individual mobile terminal containing a DGPS receiver/processorneeds a beacon receiver for receiving the DGPS signal. By way of thepresent invention, the need for a beacon receiver at each mobileterminal is eliminated. Rather, the mobile terminal need only have aDGPS processor to process the DGPS signal and correct the pseudorangescalculated by the GPS receiver. This means a reduction in the amount ofstructure present in the mobile terminal, thereby allowing for a smallersize and lower cost.

Furthermore, in the embodiment where the DGPS processor is connected tothe base station through the communications network, there is also noneed for a separate DGPS processor at each mobile terminal. This reducesthe number of DGPS processors necessary, since there is no need to havea separate processor in each mobile terminal. This allows for a furtherreduction in size and cost of the mobile terminal without sacrificingthe accuracy of position determinations made possible using DGPS. Thisconfiguration also reduces the message traffic between the network andthe terminal by allowing the position location determined by the DGPSprocessor to be sent directly to the network service requesting theinformation.

In addition, the present invention utilizes the frequency band alreadydedicated for cellular use for performing additional functions, here thetransmission of DGPS error correction data. Therefore, more efficientuse of the cellular band is made with this invention. Moreover, thecellular network provides an efficient means for having the DGPS errorcorrection information transmitted to the mobile terminal. That is, incertain instances, the mobile terminal may be located in an area wherereception of the DGPS signal from a specified reference station is notpossible. For example, the mobile terminal may not be within the properrange of the reference station. The cellular network is generallyavailable in such circumstances, and it can provide the necessary DGPSerror correction information. Therefore, the user will not lose theaccuracy attributable to DGPS.

Second Aspect

Referring now to FIGS. 8 and 9, a second aspect of the position locationsystem of the present invention will be described. This aspect addressesthe problem described above of a GPS receiver not having the requisitenumber of GPS satellites in clear view. For example, in a typicalsituation, four GPS satellites must be in clear view of the GPS receiverin order for the GPS receiver to gain an accurate fix on its location.Of course, the requisite number of GPS satellites that must be in viewmay be less than four when the GPS receiver has one or more componentsof its location already known. It is assumed for the purposes ofillustration of this embodiment that four GPS satellites are needed, butonly three of those satellites are in clear view. This may occur, forexample, when the GPS receiver is in a city setting, such as in an urbancanyon, i.e., in the shadow of a group of tall buildings, or indoors. Ofcourse, there are numerous other reasons why the GPS receiver may not bereceiving signals from the requisite number of GPS satellites, which arenot listed here.

Broadly, in the present invention, a base station of the cellularnetwork provides a pseudosatellite signal that may be used by the GPSreceiver as a replacement for a GPS satellite that is not in clear view.Furthermore, the base station provides the pseudosatellite signalswithin the cellular band, i.e., over the cellular network. In order tocarry out this aspect of the invention, the base station needs to bemodified, along with the mobile terminal, as described below.

Referring to FIG. 8, a base station 800 according to this aspect of thepresent invention is shown. Base station 800 includes a time standard805 and a GPS processing unit 810. The time standard may be anindependent reference unit such as a commercially available cesium basedreference clock. Alternatively, the cellular network infrastructure,used to synchronize base station transmissions, may also be used toprovide a time reference. GPS processing unit 810 is programmed withlocation code and is responsible for calculating pseudo GPS datarepresenting the base station's location and other related information.In addition, a C/A code unit 820 provides the encoding scheme forsignals in the civilian band. It should be noted that the base station800 does not act as a conduit for forwarding GPS satellite informationin this aspect of the invention. Rather, the base station 800 produces apseudosatellite signal that is based on the programmed code in the GPSprocessing unit 810 and the encoding scheme of the C/A code unit 820.The calculated pseudo GPS data from the GPS processing unit 810 and theC/A code generated by the C/A code unit 820 are combined by a modulator825 to produce a pseudosatellite signal. Of course, other generators mayreplace the shown GPS processing unit 810, CIA code unit 820 andmodulator 825, as long as they are able to generate a GPSpseudosatellite signal. The pseudosatellite signal produced has the samecharacteristics as a normal GPS satellite signal.

Next, in order to place the signal into proper form for transmissionover the cellular band, a converter converts the pseudosatellite signalto an appropriate radio frequency (RF) signal for transmission over thecellular band. One example of a converter is illustrated as an upconverter 830, which is supplied with a local carrier frequency, and afilter 835. In this embodiment, the filter 835 has a 2 MHz bandwidth,with a center frequency equal to the local carrier frequency.

In addition, the base station 800 may also be transmitting other signalscreated by a cellular radio 845. These represent the typical signalsthat are transmitted to the mobile terminals to provide the typicalspeech and signaling services associated with mobile phones. If bothsignals need to be transmitted together, it is desirable to set thepseudosatellite signal at a level such that it does not interfere withthe remaining cellular signals that are to be transmitted. One possibleworking parameter is to have the pseudosatellite signal broadcast at alevel which is at least twenty decibels (dB) lower than the remainingcellular signals.

To carry out this level adjustment, the RF signal is passed through alevel adjuster 840 which adjusts the amplitude level of the RF signalsuch that it is at some predetermined level below that of the remainingcellular signals transmitted through the cellular radio 845. The levelof the cellular band signals from the cellular radio 845 may vary overtime depending on the cellular system traffic or any transmit powercontrol process utilized by the cellular system. The level adjuster 840may thus change the level of the pseudosatellite signal over time(dynamically) in response to changes in cellular radio transmissions inorder to keep the pseudosatellite signal at the maximum level possible,yet not interfere with the cellular transmissions.

Thereafter, the signals from the cellular radio 845 and thepseudosatellite signal (i.e., the RF signal) are combined by a combiner850. The combined signal is adjusted by a power amplifier 855 and thentransmitted in the cellular band of the cellular network.

It should be understood that, at this point, the base station 800 hascreated the pseudosatellite signal such that it can be broadcast,together with other cellular signals, in the cellular band of thecellular network to the mobile terminal. The pseudosatellite signal maybe decoded by the mobile terminal to determine the range to the basestation. The mobile terminal is described in connection with FIG. 9below.

Turning to FIG. 9, a mobile terminal 900 according to the second aspectof the present invention is described in detail. Mobile terminal 900generally contains a cellular mobile portion 905 and a GPS receiverportion 910, and a control processing unit 940. Cellular mobile portion905 generally includes a cellular antenna 915, a low-noise amplifier920, a down converter 925, a filter 930, an IF section and base bandprocessor 935, and audio outputs 936. The GPS receiver portion 910generally includes a GPS receiver antenna 945, a low-noise amplifier950, a first down converter 955, a first filter 960, a second downconverter 965, a second filter 970, an analog-to-digital (A/D) converter975, and a digital signal processor (DSP) 980. In addition, the mobileterminal 900 contains an automatic gain controller or amplitudecontroller 990 and a switch 995. As readily understood by one ofordinary skill in the art, the particular components shown for thecellular mobile portion and the GPS receiver portion are not necessarilyexhaustive of the components contained therein, which any othercomponents are generally known in the art. Furthermore, the presentinvention contemplates substitutes of the components shown that arecapable of carrying out substantially the same functions.

It should be noted that the cellular antenna 915 and the GPS antenna 945may be formed as a single antenna that is capable of receiving bothtypes of signals. This alternative has the advantage of creating a muchmore compact mobile terminal. If the single antenna is used, then thelow noise amplifiers 920 and 950 may also be combined into a singleunit, with response designed for both cellular and GPS bands, and withoutputs to connect to down converters 925 and 955.

The cellular signal broadcast from the base station 800, which containsboth the pseudosatellite signal and other cellular signals, is receivedat the mobile terminal 900 by the cellular radio antenna 915, and isthen processed through the low-noise amplifier 920, the down converter925, and the filter 930. These last three elements work to convert thereceived signal, which has a radio frequency in the cellular band, intoa signal having a predetermined intermediate frequency (IF). Of course,any suitable converter may replace the shown low-noise amplifier 920,down converter 925, and filter 930. The IF signal is forwarded to the IFsection and base band processor 935, and depending on its contents, isforwarded to the necessary destination in the mobile terminal 900. Forexample, voice data would be forwarded to audio outputs 936, and controldata is forwarded to the control processing unit 940. In addition, theIF signal is also forwarded to the automatic gain controller 990 andthereafter to the switch 995, at input A. The automatic gain controller990 adjusts the amplitude of the pseudosatellite signal to correspond tothe GPS satellite signals, which are described below.

It should be noted that the filter 930 must be constructed to be capableof handling pseudosatellite signals, which are typically wider thanconventional cellular signals. Based on the present scheme of satellitesignals transmitting at frequencies described above—e.g., C/A code with1.023 MHz rate—the filter 930 should have the requisite width. Forexample, a width of at least approximately 2 MHz may be used. In thatregard, it is most practical if the cellular system have a bandwidththat is similar to the pseudosatellite signal bandwidth. The typicalbandwidth of IS-95 CDMA cellular signals is approximately 1.25 MHz.Therefore, a GPS pseudosatellite signal that is approximately 2 MHz widecould underlay the center of one IS-95 channel, which would mean that itwould also overlap two adjacent channels. Other options for transmissionover the IS-95 channels could be used. The IF section of 935 willtypically include a channel selection filter to separate the desiredcellular channel from the other, overlapped, cellular channels.

Moreover, an alternative to the filter 930 would be to have two filters.In such a scenario, one filter handles the pseudosatellite signals andforwards them to automatic gain controller 990. The other filter handlesthe cellular signals and forwards them to the IF section and base bandprocessor 935. With this dual filter arrangement, the channel selectionis performed together by the cellular section of filter 930 and thechannel selection filter section of 935.

Referring now to the GPS receiver portion 910, it receives satellitesignals by the GPS antenna 945 in the satellite frequency band from theGPS satellites (not shown) that are in clear view. The GPS receiverportion 910 converts those signals to correspond to the IF signalrepresenting the pseudosatellite signal; that is, having the samefrequency and substantially the same amplitude. That conversion may becarried out by routing the GPS satellite signals through the low-noiseamplifier 950, the first down converter 955, and the first filter 960.The IF signal of the GPS satellite signals is sent to the switch 995, atinput B.

The control processing unit 940 is generally responsible for controllingthe operation of the switch 995. The control processing unit 940 can beprogrammed such that it normally has the switch set to input B, but whenit determines that it is not receiving the requisite number of GPSsignals, it toggles the switch to input A to accept the pseudosatellitesignal. Thus, the control processing unit 940 utilizes as many GPSsatellites that are in view, and toggles to input A to also receive apseudosatellite signal when the requisite number of GPS satellites arenot in view of the mobile terminal. Those of ordinary skill in the artwill recognize that there are numerous alternatives to the structureshown for switching between the pseudosatellite signals and the GPSsatellite signals.

Therefore, the switch 995 chooses the signal at either input A or inputB. The selected signal is then processed by another converter—here, thesecond down converter 965 and the second filter 970—in order to convertthe signal into a baseband signal. Thereafter, the signal is convertedfrom an analog signal to a digital signal by the A/D converter 975.Next, the DSP 980 processes the signal to produce a data stream. Thecontrol processing unit 940 receives the data stream from the DSP 980and derives the position of the mobile terminal 900 in a manner known inthe art, including calculating pseudoranges that are used to determinethe position. It should be noted that the function of calculatingposition may be carried out by a separate processing unit that isseparate from the control processing unit 940.

Thus, the control processing unit 940 is configured to utilize the GPSsignals from GPS satellites that are in view, and when the requisitenumber of GPS satellites are not in view, to also utilize one (or more)pseudosatellite signals from base stations to substitute for the missingGPS satellite signal. That is, a combination of GPS satellite signalsand the pseudosatellite signals are utilized to calculate position ofthe terminal. This approach has the advantage of utilizing GPS, whichprovides the most reliable position data, to the fullest extentpossible, and only rely on the pseudosatellite signals as necessary whenGPS alone does not provide the required information.

In the above-described embodiment, the base station may be controlled tobroadcast the pseudosatellite signal as a continuous wave signal, i.e.,in a continuous manner at all times, or may be controlled to broadcastthe signal in a burst mode—i.e., broadcast, for example, twenty percentof the time. If the pseudosatellite signals are broadcast in bursts, themobile portion 905 must synchronize its reception with the bursts whichit may do through knowledge of the burst timing. In any event, themobile terminal 900 receives this pseudosatellite information wheneverit is broadcast by base station 800. Therefore, it can -be seen that thebroadcast of the pseudosatellite signal by the base station 800, and thereceipt of the pseudosatellite signal by the cellular mobile portion 905in the mobile terminal 900, is carried out without regard to whether GPSreceiver portion 910 actually has four GPS satellites in view.

However, when the GPS receiver portion 910 has four GPS satellites inview, the pseudosatellite signal is not needed. If nothing else is done,this signal is disregarded due to the switch 995. However, if desired,the control processing unit 940 may be programed to utilize thepseudosatellite signal even when the requisite number of GPS signals areavailable. In such a case, the processor central processing unit 940will have available to it five or more pseudoranges (four or more fromthe GPS satellites and one or more from the base stations). This exceedsthe number of signals necessary to carry out a position calculation, butdoes not negatively affect the position calculation made, and indeed,can improve the accuracy of the position calculation in such anover-determined system.

It should further be noted that in this embodiment of the invention, thebase station 800 need not have any GPS satellite signal receivingcapabilities itself. That is, the base station produces apseudosatellite signal independent of receiving and utilizing GPSsignals, and therefore it need not have the structure necessary forreceiving actual GPS satellite signals. Rather, the base station 800simply produces a pseudosatellite signal based on a code programmedtherein and a particular time reference, which need not be derived fromGPS.

This aspect of the invention provides an efficient scheme for producinga pseudosatellite signal, and forwarding that signal to the mobileterminal. The present invention accomplishes this by using anestablished network—the cellular network—to carry out these functions.By adding very little additional structure to already existing basestations, the need to build dedicated reference stations at strategiclocations is obviated. Furthermore, the present invention provides formore efficient use of the cellular band by utilizing it to sendadditional types of information.

Moreover, the coverage provided by the cellular network generallyincludes the strategic locations where dedicated reference stationswould otherwise be placed, including at airports and in city settings.Furthermore, the cellular network has the advantage of providingcoverage where the available number of GPS satellite signals is likelyto be insufficient, including in city settings such as in urban canyonsor indoors.

In addition, the present invention overcomes the problem that withdedicated reference stations broadcasting pseudosatellite signals, thepseudosatellite signal is stronger than actual GPS satellite signals anddrowns out the GPS satellite signals. In the present invention, thepseudosatellite signal produced and broadcast by the base station willnot interfere in any way with the actual GPS satellite signals, andtherefore will not drown out the GPS satellite signals. In fact, the twoare broadcast in completely different frequency bands. That is, the GPSsatellite signals are broadcast in the satellite band, while the basestation broadcasts the pseudosatellite signal in the cellular band.

Furthermore, when the alternative employing the switch 995 is used, thenreceipt of the pseudosatellite signal by the control processing unit 940may be completely prevented when four GPS satellites are in view of theGPS receiver portion. Again, this is an improvement over theconventional system as it enables the local control to select betweenthe pseudosatellite signals and the GPS satellite signals that areavailable.

As referred to above, the second aspect of the invention may also beexpanded to where more than one base station transmit pseudosatellitesignals, and therefore, the position of the mobile terminal 900 may bedetermined even when two (or more) GPS satellites are not in view. Forexample, two base stations may transmit pseudosatellite signals, suchthat the GPS receiver portion 910 may accurately determine its locationwhen only two GPS satellites are in its view. Indeed, any combination ofbase stations and satellites could be used. When no GPS satellites arein view, then four base stations could be used to calculate position. Ofcourse, it may also be desirable to use as many GPS satellites that arein view, along with all the available pseudosatellite signals, to obtainan over-determined result that may be more accurate.

Third Aspect

Referring now to FIGS. 10-13, a third aspect of the present inventionwill be described. As with the second aspect of the invention, thisaspect also addresses the situation where the requisite number of GPSsatellites are not in view of a GPS receiver. Broadly, in this aspect ofthe invention, the mobile terminal has the capabilities of calculatingits position using GPS, and also using the cellular networkinfrastructure, such as by the TOA or TDOA methods of determininglocation. The mobile terminal will generally use the GPS structure tocalculate position. However, when the requisite number of GPS satellitesare not in view, then it will switch to calculating position usingeither only the cellular network infrastructure, or a combination of theGPS satellite signals available and the cellular network infrastructure.When the requisite number of GPS satellites return into view, then themobile terminal switches back to relying exclusively on GPS.

First, in FIG. 10, a mobile terminal 1000 includes a GPS receiverportion 1005, a mobile cellular portion 1010, and a central processor1015. Of course, the central processor 1015 may alternatively be formedintegral with either the GPS receiver portion 1005 or the mobilecellular portion 1010, or a single processor can be used that performsthe functions of all three components. GPS receiver portion 1005includes a GPS processor 1020 for calculating position, while the mobilecellular portion 1010 contains a cellular position processor 1025 thatcomputes position using the cellular network infrastructure. Finally, asshown in FIG. 10, base stations 1030, 1035, and 1040 are part of thecellular network, and for purposes of explanation, they represent thebase stations whose transmitting vicinity includes the location of themobile terminal 1000.

Operation of the mobile terminal 1000 for determining position isdescribed in connection with the flowchart in FIG. 11. At block 1100,GPS receiver portion 1005 obtains a first fix on the location of themobile terminal, if necessary. Next, at block 1105, the first fixlocation and the locations of three (or more) nearby base stations aretransmitted to cellular position processor 1025. In this case, thenearby base stations are base stations 1030, 1035, and 1040. Cellularposition processor 1025 utilizes this information, along with thelocation of mobile terminal 1000 as determined in step 1100, todetermine the expected time difference of arrival of periodic signalsfrom base stations 1030, 1035, and 1040. For the purposes of thisexplanation, it is assumed that periodic signals are available in thecellular network, and the mobile terminal uses the TDOA process,described earlier, to determine its position from the cellular networksignals. However, the cellular position processor could alternativelyuse a TOA technique.

In particular, in order to calculate the TDOA of the periodic signalsfrom each base station 1030, 1035, 1040, the cellular position processor1025 calculates the distance between the mobile terminal 1000 and eachof the base stations 1030, 1035, and 1040, using the locationmeasurement for each item. Using the known distances and the speed ofpropagation of the radio signals, the cellular position processor 1025calculates the expected time difference of arrival of the signals fromeach base station pair. In a completely synchronized system, the time oftransmission of the periodic signals from each base station is the same,or at some specified time offset (which can be subtracted out). Bycomparing the expected TDOA with the measured TDOA, the cellularposition processor 1025 can determine the time offset of the signalsfrom each base station, and use these to correct later TDOA measurementsfor these base stations. In addition, the range, or distance to the basestation, may be calculated from the amount of time that it took for theperiodic signal to travel from the base station to the mobile terminal,i.e., the propagation delay of the signal.

Therefore, knowledge of the position of mobile terminal 1000—ascalculated by the GPS receiver portion 1005—helps to determine the timeoffset of transmission of the periodic signals from the base stations,which may be unknown in cellular systems (except, for example, some CDMAsystems implementing the IS-95 standard). This time offset oftransmission for each base station pair is stored in cellular positionprocessor 1025. Alternatively, the time offset of transmission may bestored in central processor 1015.

Thereafter, as long as the requisite member of GPS satellites are inview of GPS receiver portion 1005, the GPS receiver portion continues tocalculate position in this manner. Thus, in block 1110, the GPS receiverportion 1005 determines whether the requisite number of GPS satellitesare in view. If the requisite number of GPS satellites are in view(i.e., the answer to the inquiry is “yes”), then at block 1115, the GPSreceiver portion 1005 again calculates (or simply updates) the positionof the mobile terminal using GPS. This process is generally repeatedevery second or few seconds. Of course, the time between successivecalculations can be set to any desired amount. In addition, at block1115, the time offset of transmission of the periodic signals from eachbase station may be recalculated and updated, and stored in the cellularposition processor 1025 or the central processor 1015. Then, the nextstep will be at block 1110, where the same inquiry as to the requisitenumber of GPS satellites being in view is made.

However, if the answer to the inquiry at block 1110 is “no,” that is,the number of visible GPS satellites has dropped below the requisitelevel (usually four), then the position determination for the mobileterminal 1000 is switched over to the cellular position processor 1025.This is shown at block 1120. The switching over may be carried out bythe central processor 1015. In addition, the information of the lastknown position of the mobile terminal 1000 and each of the base stations1030, 1035, and 1040, along with the time offset of transmission of theperiodic signal from the base stations, is made available to cellularposition processor 1025 if it does not already have the information. Thecellular position processor 1025 calculates position using the periodicsignals from the base stations, as described below.

Turning to operation of the cellular position processor 1025, indicatedat block 1125, the processor 1025 will measure the TDOA for the signalsfrom the base stations. However, because the mobile terminal has likelysince moved, the measured TDOA will differ from the previous valuesafter correction for the transmission time offsets. The processor 1025may then calculate the new position using the new, corrected TDOAmeasurements as the intersection of the hyperbolic surfaces as discussedearlier.

In essence, at block 1125, the cellular position processor 1025 uses thecellular network infrastructure to determine its location, for example,by using either the TOA or TDOA methods for determining location, asdiscussed previously. Alternatively, any other method of determininglocation based on the cellular network infrastructure can be used. Ingeneral, the point is that the cellular position processor 1025calculates position based on using cellular position signals, such asthe periodic signals, rather than using GPS satellite signals. That is,the cellular position signals do not contain GPS information and ratherare independent of GPS. Thus, this aspect is unlike the second aspectwhere the pseudosatellite signals generated and forwarded by the basestation are like GPS signals.

An alternative to switching over to using a method for determiningposition of the mobile terminal based exclusively on the cellularnetwork infrastructure, such as the TOA or TDOA method, is to use acombination of the GPS satellite signals and the base station periodicsignals to determine location. This is generally indicated by dottedblock 1126 and the associated dotted lines. For example, for purposes ofexplanation, it may be assumed that only three GPS satellites are inview of the GPS receiver portion. The pseudorange determined for two ofthese satellites provides a distance measurement between each GPSsatellite and the mobile terminal. The third satellite signal provides atime reference used to calculate the range to the other two satellites.That is, the mobile terminal 1000 lies somewhere on a sphere around theGPS satellite, having a radius equal to the distance therebetween. Inaddition, calculating the distance between the mobile terminal and onebase station, the method for which is described above, provides a thirddistance measurement. Therefore, the three GPS satellites, along withthe one base station, provides three spheres whose intersectionrepresents the location of the mobile terminal. Of course, more than onebase station may be used to ensure that an accurate locationdetermination is made.

In either case—using the method according to block 1125 or block1126—once the location determination is made, the next inquiry at block1130 will be the same as the inquiry made at block 1110, i.e., whetherthe requisite number of GPS satellites are in view of the GPS receiverportion. If the requisite number of GPS satellites are not in view, thenthe same process—in either block 1125 or block 1126—is performed todetermine position. This loop between blocks 1125 or 1126 and block 1130continues until the requisite number of GPS satellites come back intoview.

When enough GPS satellites are again in view of the GPS receiverportion, then the answer to the inquiry at block 1130 becomes “yes,” andthe system proceeds to block 1135. At block 1135, determination ofposition again becomes the responsibility of GPS receiver portion 1005.This switching back to using the GPS receiver portion for calculatingposition may be carried out by the central processor 1015. In addition,at this block, determinations of the location of the nearest basestations, and their locations and time offsets of transmission of theirperiodic signals, are all updated. In sum, the system is recalibratedjust as if it started out using GPS. Thereafter, the process is returnedto block 1105, and is repeated.

In addition, it should be noted that at block 1100, if an initiallocation fix cannot be made using the GPS receiver portion 1005 becausethe requisite number of GPS satellites are not in view, then the centralprocessor 1015 can request that the cellular position processor 1025provide an initial location of the mobile terminal using theconventional TDOA method of determining location using the cellularnetwork infrastructure, as described previously herein. However, oncethe requisite number of GPS satellites come into view of the GPSreceiver portion, then the central processor 1015 will cause the systemto switch over to determining location of the mobile terminal using theGPS receiver portion.

The GPS provides good accuracy of position when its signals areavailable to the mobile terminal. This may be more accurate than methodsusing exclusively signals provided by the cellular networkinfrastructure. Thus, it can be preferable to make use of the GPS fordetermining position, and to make use of a combination of the GPS andthe cellular infrastructure when the GPS is unavailable, or partlyunavailable, to the mobile terminal. It is, of course, preferable tomake use of as many signals from both systems as are available todevelop a more accurate result than could be obtained by working witheither system exclusively.

However, a competing interest is reducing power consumption. As shown inFIG. 10, two receivers are present in the terminal. To save powerconsumption (i.e., save battery power), it may be desirable to rely onthe cellular network infrastructure to calculate position. During thistime, the GPS receiver would not be turned on. Only when the cellularnetwork infrastructure does not provide the necessary signals todetermine position, or if a GPS recalibration was needed, would the GPSreceiver be turned on and utilized to determine position. Thisalternative approach conserves battery power.

The alternative approach is shown in FIG. 12. At block 1200, a first fixis calculated. Then, at block 1210, the GPS receiver is turned off, andat block 1220, position is calculated using the cellular networkinfrastructure. At block 1230, a determination is made whether therequisite number of signals in the cellular network infrastructure areavailable for calculating position (e.g., three signals in the TDOAapproach). If so, then block 1220 is repeated. If not, then at block1240, the GPS receiver is turned on and the position is calculated usingGPS. Thereafter, the process is repeated starting at block 1210.Alternative inquiries at block 1230 include determining how long (intime) it has been since the last GPS update and/or how far (in space)the mobile terminal has moved since the last GPS update. If apredetermined amount of time has passed, such as two minutes, or themobile terminal has moved a predetermined distance, such as 100 meters,then a GPS update would be called for and the process proceeds to block1240. Of course, the rate at which GPS updates are needed will depend onthe conditions under which the mobile is operating, with less frequentupdates needed if the mobile terminal is in an area where the cellularnetwork infrastructure signals are known to be of good accuracy.

Another approach with the third aspect for determining position is thatshown in FIG. 13. At block 1300, a first fix is obtained. At block 1310,all the available GPS satellite signals are received by the GPS receiverportion. Then, at block 1320, all the available cellular positioningsignals are received. At block 1330, position is calculated using all ofthese signals. This is an overdetermined system, and the results may becombined. This may be done by weighting each location measurement in theaverage by the confidence, or expected error, in the measurements. Sucha technique is known as weighted average. This technique may be used tocombine together location measurements based on available GPS signalswith measurements based on cellular infrastructure signals withappropriate regard to the accuracy of each measurement. The weightedaverage may be calculated according to the following general expressionfor an averaged position coordinate x

x=1/wΣxn/σn²

where Xn are the measurements, σn are the expected errors, or variances,in the measurements used to weight the average, and w=Σ1/σn² is the sumof the variances used to normalize the estimate. The summations areperformed over the total number of measurements N. This choice ofweighting factors minimizes the variance of the estimate of x.

The third aspect of the invention has numerous advantages. For one, aswith the first and second aspects of the invention, this aspectefficiently utilizes the cellular network, and positioning determinationmethods available therewith, to compute the position of a mobileterminal when the GPS receiver does not have the required number of GPSsatellites in view. Furthermore, the system is efficient because itutilizes the more accurate means for determining location—GPS—wheneverthe requisite number of GPS satellites are available.

In addition, there is another advantage to switching back to GPS afterthe mobile terminal has been using the cellular network infrastructurefor some period of time to determine location. By switching back to GPS,and recalibrating the system, the negative effect of multi-path problemsassociated with using the TOA and TDOA methods for determining locationis reduced. That is, after some period of time of using the TOA or TODAmethods to determine location, the mobile may have moved out of theregion for which the last calibration is appropriate. Recalibrating withthe GPS enables any errors due to the use of the cellular infrastructuresignals to be again determined and corrected.

By switching back to using GPS as soon as it becomes feasible, the TOAor TDOA methods will be used for the absolute minimum amount of time,which limits the effect of errors due to multi-path problems. Using boththe GPS and cellular signals, an estimate may be made of the multi-pathpropagation errors in the cellular signals.

Furthermore, another problem associated with calculating position basedon cellular network infrastructure is clock drift of the clock in themobile terminal. That clock drift can result in erroneous time—andtherefore location—measurements. By switching back to GPS as soon as itbecomes available, the time in which the TOA or TDOA methods are used isrelatively short. Thus, the amount of error due to clock drift, whichincreases the longer the cellular network infrastructure methods areused, can be minimized. In addition, by measuring the rate of clockdrift in the mobile terminal using the GPS time information while theGPS signals are available, a software routine may be implemented, forexample, in the central processor, to compensate for this clock driftwhen GPS is not available at a subsequent point in time.

Fourth Aspect

A fourth aspect of the invention may be described in connection with thestructure shown in FIG. 10, and FIGS. 14-15. In a standard IS-95 CDMAsystem, the pilot signal component of the CDMA cellular base stationsignal can be used to augment and improve the accuracy and availabilityof position location using GPS. The CDMA cellular signal can provide thefunctional equivalent of the GPS pseudorange. The advantage is that thisapproach requires very little adaptation of the base station, forexample, compared to the second aspect described above. The CDMA pilotsignal component, such as used in the IS-95 standard, has the advantageof being transmitted continuously from each base station at a constantpower level that enables it to typically be received by a mobileterminal from more than one base station. The pilot signal includesperiodic signals that are transmitted at specified times with specifiedoffsets. Thus, the time difference of arrival of two pilot signals fromtwo base stations may be readily measured by the mobile terminal. TheIS-95 standard signalling and control process also includes methodswhereby the terminal may be instructed to measure the TDOA of the pilotsignals it can receive and to report these measurements to the CDMAcontrol process. Other facets of the IS-95 pilot signals and thestandard transmissions enable a round trip delay (“RTD”) measurement tobe made which may be used to determine the mobile's range, or distance,from the serving base station. Either the RTD measurement or a pluralityof TDOA measurements may be used to obtain a cellular distancemeasurement, which represents the distance between the mobile and theserving base station.

In this approach, a base station, such as base station 1035, transmitstiming signals to the terminal. In fact, this is already being done aspart of the regular cellular system transmissions such as according toIS-95 or GSM standards. The terminal will receive the signals at somelater time due to the delay in transmission. The terminal will extractthe timing signals from tie base station, though those signals will beoffset in time due to the propagation delay. Further, the terminal musttransmit back to the base station at defined times with respect to itsreceipt of the timing signals from the base station. For example, theterminal may be required to respond within plus or minus one microsecond(±1 μs) of its specified transmission timing. The base station thenreceives the return signals from the terminal at some later time due todelay in return transmission. The total delay measured at the basestation is the RTD. The radius of the sphere—representing the distancebetween the base station and the terminal—is one half the RTD times thespeed of radio signals in air. This measurement may be generallyreferred to as a cellular distance measurement. The cellular distancemeasurement, like the cellular position signal of the third aspect, doesnot contain GPS information and rather is independent of GPS.

Therefore, this distance measurement can be used as a substitute for aGPS signal when the requisite number of GPS satellite signals are not inview, or to supplement the information available from GPS so that bothGPS and cellular signals are used together in order that a more accurateposition determination may be made than by either system operatingalone. The advantage of this approach is that the only change to thebase station is the addition of the RTD determination, which can be doneby a processor. FIG. 14 shows a typical base station 1400 with acellular transceiver 1410 for performing conventional functions and aRTD processor 1420 that determines RTD.

However, errors are present in the above position; estimate, largely dueto the terminal's internal delays and multi-path propagation. While theuncertainty is generally less than 1 microsecond, this translates intoan uncertainty in distance of about 150 meters.

To compensate for this error, GPS can be used. This approach is depictedin FIG. 15. When a requisite number of reference GPS satellites are inview, the position of the terminal can be determined. The reference GPSsatellites need not be the same GPS satellites that the mobile makes useof at some later point in time when the requisite number of GPSsatellites are not in view. This determination is shown in block 1500.The position may be sent to a position processor which may be locatedeither in the mobile terminal (such as 1025 in FIG. 10) or may be aserver in the network which operates to calculate positions forterminals using the cellular and communications networks. This is shownat block 1510. In addition, it is noted that the locations of the basestations are known. At block 1520 the position processor determines theexpected signal timing using information on the position of the basestation and the position of the mobile as determined using GPS. Forexample, in the case of a system using the RTD measurement processdescribed earlier, the expected RTD would be calculated. Alternatively,for a system using the TDOA measurement process described earlier, theexpected TDOA would be calculated. Then at block 1530 a measurement ismade of the actual signal timing, for example the RTD or the TDOA. Atblock 1540, the difference between the expected and the measured valuesis determined. The difference is stored, at block 1550, as a cellularcorrection term (i.e. a RTD or TDOA correction term) for later use bythe position processor. If the position processor is located in themobile terminal, then the RTD measurements, which are made in the basestation, may be sent to the mobile terminal using the standard messagesignalling facilities of the cellular system. Similarly, if the positionprocessor is connected to the communications network, the TDOAmeasurements made in the terminal may be sent to the position processorusing the standard message signalling facilities of the cellular system.Of course, to determine the position to the best accuracy, the positionprocessor may make use of both TDOA and RTD measurements in itscalculations and need not be restricted to a single measurement type.

Thereafter, when the requisite number of GPS satellite signals are notin view at some later point in time, and position is calculated based inpart using the RTD measured at the base station, or the TDOA measured atthe terminal, the correction term can be used to reduce the error due tothe unknown delay in the terminal as seen at the serving base station.It is contemplated that the position location calculation be done by thenetwork server, or in a suitable processor in the terminal (such ascentral processor 1015 or position processor 1025 in FIG. 10). In thealternative, the cellular position processor 1025 in the mobile terminalcould measure TDOA of the cellular pilot signals and use these tosupplement the GPS position information.

It should be noted that other systems (in addition to the IS-95 CDMAtechnique discussed) exist that are capable of measuring the RTD, suchas the GSM (European standard) that utilizes TDMA (Time DivisionMultiple Access) cellular techniques. The same concept of utilizingcellular distance measurements as substitutes for missing GPS satellitesignals may be readily applied in such systems. Typically GSM systemsare operated with unknown time offsets in the transmissions from eachbase station. In this case the correction terms, determined through theuse of the GPS calibrations, also compensate for the unknown timeoffsets of the GSM base station transmissions.

It should be understood that while the four aspects of the positionlocation system of the present invention have each been described, andmay be used, separately, two or more aspects may be combined in a singleposition location system. For example, the DGPS error correctioninformation may be provided to the mobile terminal through the cellularnetwork according to the first aspect of the invention, and in that samesystem, a base station of the cellular network may also provide apseudosatellite signal. In such a system, the control processing unit940 would utilize the error correction data and correct the GPSpseudoranges to obtain corrected GPS ranges. Thus, the DGPS processorwould essentially be part of the control processing unit. Of course, aseparate DGPS processor could be used instead.

Alternatively, the first and fourth aspects could be combined. DGPSerror correction data would be provided to the mobile terminal and thebase station could provide the CDMA cellular signal that provide thefunctional equivalent of the GPS satellite signal. Another alternativeis that the DGPS error correction information be provided through thecellular network infrastructure in a system that also switches betweenusing GPS and the cellular network infrastructure to determine location.

Furthermore, the position location system may have the second, third,and fourth aspects of the invention available, and the system merelydetermines which aspect to utilize when the requisite number of GPSsatellites are not in view of the GPS receiver. The selection of thedesired approach could be carried out, for example, by a processor suchas the central processor 1015 of the mobile terminal, shown in FIG. 10.Finally, all four aspects may be combined in a single position system aswell. Therefore, the present invention is not limited to using only oneaspect of the invention in a single position location system. Rather,two or more aspects of the invention can be used in the same positionlocation system.

In addition, changes to the structures as presented above do not departfrom the scope of the present invention. For example, a mobile terminalmay contain both the GPS and cellular portions in the same mobile unit,or each component can be housed separately with a relevant means forcommunication between the two. In that regard, the term “mobileterminal” may refer to a terminal containing either a GPS receiver or acellular mobile terminal, or both. Furthermore, many of the individualcomponents may be combined into a single unit with other components. Forexample, the central processor in the third aspect of the invention maybe contained in either the GPS receiver portion or the mobile cellularportion.

Furthermore, substitute components that provide substantially the samefunctions as those disclosed also do not depart from the scope of theinvention. For example, alternate structures known in the art forconverting a received signal into an intermediate frequency (IF) may beused rather than the structure disclosed herein. Finally, the generalcomponents shown for the mobile terminals, base stations, and MTSO donot necessarily indicate that this is the only structure present inthese items. Rather, the illustration of certain components is intendedfor an easier understanding of the present invention. Finally, while theconnections in the Figures, for example between the DGPS processor andthe GPS receiver in FIG. 5, are shown as electrical connections, itshould be understood that other connections are possible, such asoptical connections.

We claim:
 1. A position location system for determining a geographicposition comprising: a cellular antenna that receives a cellular signalhaving a frequency in a cellular frequency band, wherein said cellularsignal contains global positioning pseudosatellite data; a firstconverter connected to said cellular antenna, wherein said firstconverter converts said received cellular signal into a firstintermediate frequency signal having a first frequency; a globalpositioning system receiver antenna that receives global positioningsystem satellite signals having frequencies in a satellite frequencyband from a plurality of global positioning system satellites; a secondconverter connected to said global positioning system receiver antenna,wherein said second converter converts said received global positionsystem satellite signals into a second intermediate frequency signalhaving a second frequency, wherein said first frequency of said firstintermediate frequency signal is substantially equal to said secondfrequency of said second intermediate frequency signal; a selectorconnected to said first converter and to said second converter, whereinsaid selector selects one of said first intermediate frequency signaland said second intermediate frequency signal; and a control processingunit connected to said selector, said control processing unit configuredto utilize said first intermediate frequency signal and said secondintermediate frequency signal when a requisite number of globalpositioning system satellites are not in view of said global positioningsystem receiver antenna, to calculate global positioning systempseudoranges, wherein said selector selects only one of said first andsecond intermediate frequency signals to be provided to said controlprocessing unit at any particular instant in time, wherein said selectoris controlled by said control processing unit such that said selector isnormally set to a first position, the first position providing only saidsecond intermediate frequency signal to said control processing unitwhen the requisite number of global positioning system satellites are inview of said global positioning system receiver antenna, and whereinsaid selector is set to the first position for a first period of timeand then set to a second position for a second period of time to providesaid second and first intermediate frequency signals separately to saidcontrol processing unit when the requisite number of global positioningsystem satellites are not in view of said global positioning systemreceiver antenna.
 2. The position location system of claim 1, furthercomprising a mobile terminal that houses said cellular antenna, saidfirst converter, said global positioning system receiver antenna, saidsecond converter, said selector, and said control processing unit,wherein said cellular signal is generated internally within a cellularnetwork base station without using any global position satellite dataoutput by the global position system satellites.
 3. The positionlocation system of claim 2, further comprising: a plurality of cellularnetwork base stations positioned at predetermined locations; and acommunications network that communicates with said plurality of cellularnetwork base stations, wherein at least one of said plurality ofcellular network base stations comprises a generator that generates saidcellular signal containing said global positioning systempseudosatellite data, and wherein said one of said plurality of cellularnetwork base stations transmits said cellular signal to said mobileterminal.
 4. The position location system of claim 1, wherein saidcellular signal further contains differential error correction data, andwherein said control processing unit is further configured to correctsaid global positioning system pseudoranges using said differentialerror correction data, to obtain corrected global positioning systemranges.
 5. The position location system of claim 1, fierier comprisingan amplitude controller connected to said first converter, wherein saidamplitude controller adjusts an amplitude of said first intermediatefrequency signal before said first intermediate frequency signal isforwarded to said selector.
 6. The position location system of claim 1,wherein said control processing unit controls said selector such thatsaid selector selects said first intermediate frequency signal at afirst instant in time, and said selector selects said secondintermediate frequency signal at a second instant in time different fromsaid first instant in time.
 7. A position location system fordetermining a geographic position comprising: a cellular antenna thatreceives a cellular signal having a frequency in a cellular frequencyband, wherein said cellular signal contains global positioningpseudosatellite data; a first converter connected to said cellularantenna, wherein said first converter converts said received cellularsignal into a first intermediate frequency signal having a firstfrequency; a global positioning system receiver antenna that receivesglobal positioning system satellite signals having frequencies in asatellite frequency band from a plurality of global positioning systemsatellites; a second converter connected to said global positioningsystem receiver antenna, wherein said second converter converts saidreceived global position system satellite signals into a secondintermediate frequency signal having a second frequency, wherein saidfirst frequency of said first intermediate frequency signal issubstantially equal to said second frequency of said second intermediatefrequency signal; a selector connected to said first converter and tosaid second converter, wherein said selector selects one of said firstintermediate frequency signal and said second intermediate frequencysignal; and a control processing unit connected to said selector, saidcontrol processing unit configured to utilize said first intermediatefrequency signal and said second intermediate frequency signal when arequisite number of global positioning system satellites are not in viewof said global positioning system receiver antenna, to calculate globalpositioning system pseudoranges, wherein said selector selects only oneof said first and second intermediate frequency signals at anyparticular instant in time, wherein said cellular signal contains theglobal positioning system pseudosatellite data only at predeterminedtime frames, and wherein said selector is synchronized to select saidfirst intermediate frequency signal only during said predetermined timeframes when said control processing unit determines that the requisitenumber of global positioning system satellites are not in view of saidglobal positioning system receiver antenna.
 8. A method for determininga geographic position of a mobile terminal comprising the steps of:receiving global positioning system satellite signals having frequenciesin a satellite frequency band at the mobile terminal from a plurality ofglobal positioning system satellites that are in view of the mobileterminal; receiving a cellular signal having a frequency in a cellularfrequency band at the mobile terminal, said cellular signal beinggenerated internally within a cellular network base station withoutusing any global position satellite data output by any of said pluralityof global position system satellites; calculating a cellular distancemeasurement representing a distance between the mobile terminal and thecellular network base station using said cellular signal; correctingsaid cellular distance measurement using a cellular correction term, toobtain a corrected cellular distance measurement; calculating theposition of the mobile terminal using only said received globalpositioning system signals when a requisite number of global positioningsystem satellites are in view of the mobile terminal; and calculatingthe position of the mobile terminal using said received globalpositioning system signals and said corrected cellular distancemeasurement when the requisite number of global positioning systemsatellites are not in view of the mobile terminal.
 9. The method ofclaim 8, wherein said cellular correction term is a round trip delaycorrection term that is calculated by the steps comprising: determiningan expected round trip delay between the mobile terminal, whose positionis determined using reference global positioning satellites, and saidcellular network base station, whose position is known; determining anactual round trip delay between the mobile terminal and said cellularnetwork base station; calculating a difference between said expectedround trip delay and said actual round trip delay; and storing thecalculated difference as said round trip delay correction term.
 10. Themethod of claim 8, wherein said received cellular signal is a codedivision multiple access cellular signal.
 11. The method of claim 8,wherein said received cellular signal is a time division multiple accesscellular signal.
 12. The method of claim 8, wherein said cellularcorrection term is a time difference of arrival correction term that iscalculated by the steps comprising: determining an expected timedifference of arrival between the mobile terminal, whose position isdetermined using reference global positioning satellites, and saidcellular network base station, whose position is known; determining anactual time difference of arrival between the mobile terminal and saidcellular network base station; calculating a difference between saidexpected time difference of arrival and said actual time difference ofarrival; and storing the calculated difference as said time differenceof arrival correction term.